EAS (electronic article surveillance) systems are often installed in a variety of challenging and difficult environments where various noise sources may interfere with the operation of the system. EAS systems are typically installed at retail or other outlets for the purpose of inventory control, to detect the presence of articles exiting a store or other area and to prevent theft or unauthorized removal of such articles. Articles are tagged with appropriate security devices which emit signals detected by the EAS system when an article is in the detection range of the system, (also referred to as the interrogation zone), triggering an alarm. Under normal circumstances, the security device is removed by store personnel at the point of sale to prevent such an alarm from being routinely and unnecessarily triggered.
Some of the principal features and operations of EAS systems, such as those discussed in the present invention, are as follows:
(1) A transmission circuit generates a burst signal at a given operating frequency. This signal is radiated into the interrogation zone through which the tagged article might pass.
(2) If a tag is present in the interrogation zone, the tag will respond to the transmission field by resonating or oscillating in sympathy with the stimulus field, analogous to a tuning fork being struck by an impulse.
(3) Immediately following the cessation of the transmission burst, the antennas in the system will pick up the resonant response of the tag. This is a weak signal, necessitating high sensitivity to low-level signals, immediately following the transmission of a much larger stimulus signal.
(4) If this weak resonant tag response is detected, the system will produce an alarm indicating to store personnel the presence of a tag (i.e. a tagged article) passing through the interrogation zone.
In order to be effective, the EAS system must be sufficiently sensitive to the weak tag response signal following the transmission burst, but not be sensitive to electrical or other noise present in the environment which may be picked up by the system and thereafter produce false alarms.